1. Field of the Invention
The present invention relates to a process for producing a plasticized vinylidene chloride-vinyl chloride copolymer. More particularly, it relates to a process for producing a plasticized vinylidene chloride-vinyl chloride copolymer having homogeneous quality and containing less residual monomer without a blending step.
2. Description of the Prior Art
Vinylidene chloride-vinyl chloride copolymers especially the copolymers having 65 to 95 wt.% of vinylidene chloride component have characteristic of superior gas barrier property, but have inferior fabricatability. Therefore, in usual, the fabricatability should be improved by blending a plasticizer or a stabilizer in a copolymer powder. The resulting compounds are melt-extruded and quenched to obtain compositions which have superior fabricatability, (for example, stretch-orientation can be easily performed at about room temperature) and which have been fabricated into films, bottles, yarns etc. and have been widely used for food packages and other fields.
In usual, a suspension polymerization and an emulsion polymerization has been employed for producing copolymers. In a suspension polymerization, an emulsifier is not incorporated as different from an emulsion polymerization, whereby excellent heat stability of the resulting polymer can be expected and hygroscopic whitening can be prevented and a post-treatment can be simplified. Therefore, a suspension-polymerization has been widely applied.
However, suspension-polymerized polymers have larger particle diameter in comparison with emulsion-polymerized polymers whereby residual monomers of vinylidene chloride and vinyl chloride remain in the suspension polymerization are not easily removed.
Recently, the residual monomer problem has been seriously considered in plastics used for food packages. It has been seriously desired to minimize the residual monomers in the polymers to be negligible even though the residual monomers are vinylidene chloride, vinyl chloride and other comonomers. Vinylidene chloride has a boiling point of 30.degree. C. which is higher than the boiling point of vinyl chloride, whereby it is difficult to remove completely vinylidene chloride monomer by a conventional post-treatment after the suspension-polymerization. Therefore, vinylidene chloride monomer is removed in a blending step for blending a plasticizer and other additives at high temperature whereby there are certain disadvantages of requiring a long blending time to deteriorate thermal stability of the polymers.
It has been known to reduce contents of the residual monomers by adding a plasticizer in a slurry obtained by a suspension-polymerization of vinylidene chloride and vinyl chloride in Japanese Unexamined Patent Publication No. 101288/1977.